With the increase in environmental oversight, operators of power plants are pushing to discover new and better ways to remediate potential pollutants which are the byproducts of the power generation process. A variety of approaches have been developed for removal or mitigation of such byproducts resulting from coal fired power plants. One known approach is the use of dry sorbent injection (DSI) systems to reduce acid gas levels, such as such as sulfur dioxide (SO2), sulfur trioxide (SO3), sulfuric acid (H2SO4), and hydrochloric acid (HCl). DSI involves the addition of an alkaline material (such as sodium bicarbonate, hydrated lime, or trona) into various locations of the power plant system such that the acid gases react with the alkaline sorbents to form solid salts which are removed via a particulate control device.
While DSI is a cost effective control solution, it is not without its own processing challenges. For instance, certain sorbent materials are prone to clumping or agglomerization, while some sorbent materials (e.g., trona) are known to require milling in order to increase the surface area for absorption and to be more cost effective. For a variety of reasons (e.g., superior flow properties and predictability of particulate size), the use of on-site milling for certain sorbent particulate is preferred.
One particularly effective method of on-site milling is the use of an in-line mill using a pneumatictransport or conveying system, which provides a superior reduction of SO3 stack emissions when injecting trona. This approach, however, typically requires the use of a bearing assembly to support and facilitate the milling process. The use of a bearing assembly, in turn, creates problems in that the bearing assembly requires oil or similar lubricant for operation, but the nature of the in-line mill requires an operational clearance between the bearing assembly casing and the running surface (i.e., such that the shaft or similar moving components can rotate). Thus, the use of such milling system requires a bearing seal such that the lubricant is kept apart from the fluid of the milling area (and vice versa). A wide variety of seals are known for such processes.
One class of bearing seals are contacting seals, such a lip seal or more complex designs as in the case of a face/mechanical seal. Contacting bearing protectors such as lip seals often require lubrication at the contact points of rotational surfaces. In high shaft rotational speed applications, excessive heat can be created in absence of sufficient lubrication. Such a lubrication requirement can make this approach unacceptable for a milling system which requires high speed rotation of the shaft assembly.
By contrast, non-contacting bearing protectors can be of repeller or labyrinth configuration. A labyrinth seal may be composed of many grooves or knives so that the fluid has to pass through a long and difficult path to escape between the operating surfaces. Sometimes such knives exist on the outer and inner portion of the mating surface. These knives or grooves produce long characteristic path and pressure resistance which slows leakage. For labyrinth seals on a rotating shaft, a very small clearance must exist between the knife tips of the labyrinth and the running surface.
Unfortunately, in milling applications which require the processing of particulate in a pressurized fluid, the particulate often penetrates the close rotating clearances of conventional labyrinth seal technology, leading to blockage and rotational seizure. An air purge system can be used to overcome particulate entry, but skepticism has developed around the effectiveness of such an approach, since while some of the air acts to blow away the powder from the bearing seal entry point, air is also directed into the bearing chamber. The consistent injection of pressurized fluid or air proximate to the seal can force particulate and similar contaminants into the oil or other lubricant-based environment supporting the bearing assembly. Such an outcome can result in undue wear or premature replacement of the bearing assembly and/or replacement or failure of the mill, with the consequential degrading of the efficacy of the contaminant remediation process.
In other words, an in-line mill system needs to have a seal which minimizes or precludes leakage of pressurized fluid into the lubricant chamber of the bearing assembly, while at the same time preventing an excessive fluid or air pressure from the process side forcing particulate into the bearing assembly.
Thus, the present state of the art reflects a need for a system which reliably mills entrained particulate entrained in a pressurized fluid (such as sorbent particulate in a pneumatic conveyance system) in an in-line mill configuration without creating undue wear or buildup in the bearing assembly, as such wear may increase maintenance costs and failure risks and decrease the efficiency of the system.